As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having a high energy density and a high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
However, the secondary battery has a problem in that the charge and discharge efficiency of the secondary battery due to an instantaneous high current is low although the secondary battery has a high energy density. In order to solve the above-mentioned problem of the secondary battery, research and development has been recently made on a technology for systematically coupling an electric double-layer capacitor (EDLC) to a conventional lithium ion polymer battery (LIPB).
On the other hand, a global system for mobile communication (GSM), which is widely adopted by European mobile phone manufacturing companies, requires the supply of a high current for a short period of time during a discharge cycle. When the GSM is adopted, however, the capacity of a conventional secondary battery is seriously reduced during the high-rate charge and discharge of the secondary battery, and therefore, the solution to this problem is very urgent.
Generally, a capacitor is a device that accumulates electric charges during the application of voltage to the capacitor. The capacitor exhibits high output characteristics. A representative example of the capacitor is an electrochemical capacitor, which may be classified as an electric double-layer capacitor (EDLC) or a pseudo capacitor. The electric double-layer capacitor is a device that stores electric charges by charging ions on an electrolyte and electrons on an electrode at an electric double-layer formed at the interface between the electrode and the electrolyte. The pseudo capacitor is a device that stores electrons adjacent to the surface of an electrode material using a Faraday reaction.
The electric double-layer capacitor includes an equivalent circuit constructed in a structure in which a double-layer capacitance and an equivalent series resistance (ESR) are connected in series with each other. In this case, the double-layer capacitance is proportional to the surface area of the electrode, and the ESR is the sum of the resistance of the electrode, the resistance of an electrolytic solution, and the resistance of the electrolyte in pores of the electrode. The instantaneous high-output characteristics of the electric double-layer capacitor are excellent; however, the energy density and storage characteristics of the electric double-layer capacitor are poor as compared to the conventional secondary battery.
A hybrid type battery, constructed in a structure in which the capacitor, having the above-described characteristics, is coupled to a secondary battery, has an increased instantaneous output and a high energy density. Generally, however, the hybrid type battery is manufactured by interconnecting the secondary battery and the capacitor, which are separated from each other. Consequently, the manufacturing process of the hybrid type battery is complicated, and the installation space of the hybrid type battery is increased, with the result that the minimization of the battery is not accomplished.
In this connection, there has been proposed an electricity storage apparatus constructed in a structure in which a capacitor is simply included in a battery having an acid electrolyte, especially an electrochemically active polymer, applied to at least one side of a cathode and an anode as an electrode active material. However, it is structurally difficult to manufacture this apparatus, with the result that it is difficult to mass-produce the apparatus. Furthermore, the acid electrolyte is used as the electrolyte. Consequently, when electrodes for a secondary battery, such as lithium cobalt oxide (LiCoO2) and graphite, are applied to the apparatus, the cycle degeneration of the apparatus occurs.
On the other hand, a carbon material is generally used as the electrode material for the electric double-layer capacitor. In order for the carbon material to exhibit excellent electric double-layer capacitor characteristics, it is required that (i) the carbon material includes a large number of pores to provide a large specific surface area, (ii) the carbon material has a high conductivity, and therefore, when the electrode is manufactured with the carbon material, the carbon material has a low electrode resistance, and (iii) the pores of the carbon material have a sufficiently large size, the connectivity between the pores of the carbon material is excellent, whereby the surfaces of the pores are easily wetted, by the electrolyte solution, to form a wide electric double layer, and the movement of the electrolyte ions is easy, whereby the charge and discharge are rapidly carried out.
For the conventional capacitor, active carbon is used as the electrode material in order to satisfy the above-mentioned conditions. However, the active carbon is relatively expensive, with the result that the manufacturing costs of the capacitor are increased, and therefore, it is difficult to mass-produce the conventional capacitor.
Consequently, there is a high necessity for a technology that is capable of fundamentally solving the above-mentioned several problems.